As set forth in that application, oil well drilling techniques require recirculation of drilling mud. The mud that is normally used in the drilling process serves as a lubricant for the drill bit, washing away cuttings as the bit progresses through the earth and carrying the cuttings in a suspended form back to the surface. The drilling mud is made of a number of products which include abrasive materials and is relatively heavy or thick. Being laden with a number of particles and under substantial pressure, drilling mud very often cuts valves and valve seats of control valves interposed in the mud flow system. The present invention is a flow control choke for use in a mud system. In particular, it is a flow control choke which is interposed downstream from the pump before the mud is delivered to the mud pits to control the back pressure in the drill string, itself, and to control the rate of delivery of mud to the mud pits.
Various and sundry approaches have been used to the fabrication of drilling mud flow control chokes or valves. The present apparatus is deemed to be an improvement over known structures in that it offers an adjustably positioned, tapered plug cooperating with a tapered seat, both of the members being formed of a hardened material. It has the feature of fitting the tapered plug into the tapered seat without contact against the sidewalls. Moreover, the throttling surface is not the surface that provides actual closure in the event that the valve is fully closed. Perpendicular shoulders upstream from the tapered surfaces serve this purpose, yielding the advantage that flow through the tapered plug and seat does not require full closure thereof on the stream and the consequential erosion that occurs in routine operation. The apparatus is exposed to mud flow heavily laden with particles which might damage, harm or otherwise destroy the equipment. The surfaces which provide the throttling effect, therefore, serve only that purpose, while the valve and seat surfaces which provide full shutoff do not serve dual purposes.
One advantage of the present invention is thus the ability of the apparatus to provide continual throttling to mud flow with surfaces which are never required to contact one another. This prevents closure of these surfaces against one another with the consequential damage that occurs should particulate cuttings be trapped between them. They close toward one another to achieve the throttling so attractive in the present invention, but they do not contact. Moreover, these surfaces are concentric to one another so that, at a given spacing or throttling position, the flow is controlled even though the plug serving as a valve may be partly worn away. Erosion of the components must be excessive and substantially total before it actually modifies the gap or spacing through the throttling flow controller.